Metabolism of 14C-arachidonate was investigated in rat isolated lungs perfused via the pulmonary circulationnn with Krebs solution. Only 10% of the radioactivity derived from an infusion of 14C-arachidonate through the pulmonary circulatio of rat isolated lungs appeared in the effluent by 10 minutes. At 10 min, the major component of effluent radioactivity and 20-40% of that retained in lung was unchanged arachidonate. Between 10 and 20 min of perfusion, a further small amount of radioactivity was lost in lung effluent and at 20 min the retained radioactivity showed a decrease in the proportion present as free arachidonate. Between 20 and 60 min, there was no further loss of radioactivity in effluent and no further change in the distribution in lung. Addition of albumin to the Krebs solution perfusate during the infusion of 14C-arachidonate increased effluent radioactivity to 80%, but albumin added after 10 min only caused the efflux of a small amount of radioacitivyt (10%). Treatment of labelled lung at 20 min with the calcium ionophore A23187 released biologically active metabolites of arachidonate but very little radioactivity. Metabolism of arachidonate, either during the infusion or after retention in lung, in rat lung was closer to that in human lung than to that in guinea-pig lung.